1. Field of the Invention
This invention relates to a semiconductor laser drive circuit in a laser beam printer using a semiconductor laser beam.
2. Background of the Invention
A so-called "laser beam printer" is being extensively employed in which a charged photo-sensitive material is scanned with a laser beam modulated with printing input data on characters, pictures, symbols, etc. to form a latent image. The latent image thus formed is developed, transferred and fixed for obtaining a hard copy of the printing input data. A printer of this type using a semiconductor laser beam has been practically used already.
FIG. 1 is a block diagram outlining the arrangement of a laser beam printer using a semiconductor laser beam. In FIG. 1, a feedback circuit 11 (or constant current circuit) includes a photo-detector 12. A modulating circuit 13 receives its output and provides its modulated output to a semiconductor laser 14 (laser diode). In the laser beam printer thus organized, a laser beam LB modulated with printing input data MD is outputted by the semiconductor laser 14.
The printing input data MD are applied to the modulating circuit 13 directly from a computer or through a memory medium such as a magnetic tape or a magnetic (or optical) disk with the aid of an interface.
In the modulating circuit 13, a current is subjected to an ON/OFF modulation with the printing input data MD. The semiconductor laser 14 produces a laser beam LB according to the current thus modulated.
On the other hand, the photodetector 12 detects the power of the laser beam LB. When the power is lower than a predetermined value, the feedback circuit 11 is activated to increase the power to the predetermined value. That is, the constant current feedback circuit 11 and the photodetector 12 form an automatic power control (APC) for stabilizing the power of the laser beam LB.
Further in FIG. 1, a laser beam scanning section 15 receives the modulated laser beam LB and the scanned laser beam is taken by an electrophotographic processing section 16. A hard copy output section 17 provides the final paper output.
The laser beam scanning section 15 has a polygon mirror, which is rotated by a drive motor at a constant speed to reflect the laser beam LB so that a photosensitive drum is scanned with the laser beam LB.
The electrophotographic processing section 16 has a charged photosensitive drum, a charging unit, a discharging unit, and a developing unit. In the electrophotographic processing section 16, an electrostatic latent image is formed on the photosensitive drum by the laser beam LB scanning it, and is developed by the developing unit.
The hard copy output section 17 includes a transferring unit, a sheet supplying and removing unit, and a fixing unit. In the hard copy output section 17, the image developed on the photosensitive drum is printed on a printing sheet, that is, is produced as a hard copy of the printing input data.
The printer using the semiconductor laser beam is advantageous in that it is small in size and low in manufacturing cost. However, it is disadvantageous in that, since the semiconductor laser is employed, the operation is liable to be adversely affected by the variation of the supply voltage or the ambient temperature. Thus, the power of the laser beam is unstable.
The disadvantages of the printer will be described with reference to FIGS. 2 and 3 in more detail.
FIG. 2 is a circuit diagram showing the feedback circuit 11, the photodetector 12, the modulating circuit 13, and the semiconductor laser 14 in more detail.
The feedback circuit 11 comprises a p-i-n photodiode 12a with fast response and a constant current circuit including an operational amplifier 11a which receives as differential input voltages a reference signal V.sub.r and a detection voltage V.sub.s which a processing circuit 11c outputs in response to the current detected by the photodiode 12a. An amplifying transistor 11b receives at its base the output of the operational amplifier 11a.
The processing circuit 11c operates to control printing gradations and to adjust the power of the laser beam LB affected for instance by the ambient temperature.
The modulating circuit 13 is made up of a switching transistor 13b to the base of which input printing data MD are applied through an inverter 13a.
The semiconductor laser 14 employs a laser diode 14a which is connected through a resistor 19 connected to the collector of the transistor 11b which is emitter-grounded through a resistor 18.
FIG. 3 is a graphical representation indicating one example of the output characteristic of the laser diode 14a, in which the horizontal axis represents the drive current I to the resistor and the vertical axis represent beam output power P. The output characteristic was obtained at an ambient temperature of 25.degree. C. Therefore, when the ambient temperature is lower than 25.degree. C., the output characteristic curve is shifted to the left and when it is higher than 25.degree. C., the output characteristic curve is shifted to the right.
In the circuit thus organized, the drive current I in the laser diode 14a can be characterized as follows; EQU I=(V.sub.cc -V.sub.s)/R
Where R is the resistance of the resistor 18.
As is apparent from the above equation, the drive current I changes with the supply voltage V.sub.cc. This is because the reference voltage V.sub.r of the operational amplifier 11a depends on the supply voltage V.sub.cc. The supply voltage V.sub.cc is, in general, obtained from a constant voltage circuit. Therefore, a great change of the drive current is rarely caused. However, even a small change of the drive current is not preferable because, owing to the output characteristic of the laser diode 14a, the small change in the drive current I appears as a large variation in the laser beam output P.
This relation can be understood from FIG. 3. When the drive current I changes slightly, as much as .DELTA.I, then it appears as a large variation .DELTA.P in the laser beam output.
The variation of the laser beam output P out of a predetermined range not only lowers the quality of the hard copy, but also causes erroneous modulation. Therefore, it is desirable to stably maintain the supply voltage to thereby suppress the variation of the drive current I as much as possible. However, since the constant voltage circuit itself, being affected by the ambient temperature and electrical noises, changes its output voltage, the above described problem is still involved.
In the above-described semiconductor drive circuit, while the laser beam is being modulated, a voltage developed across the laser diode 14a and the resistor 19 changes greatly, thus affecting the feedback circuit 11. That is, the voltage obstructs the supply of the constant current by the feedback circuit 11, with the result that the above-described difficulty takes place.